This invention relates to the world-wide problem of hunger and of increasing yields of crops to feed an ever increasing human and food animal population. This invention also relates to the problem of controlling diseases in plants. A large proportion of the world's food supply is lost every year due to plant diseases which reduce food yields by killing plants, reducing numbers of fruits and vegetables from infected plants, and by retarding the growth of crop plants.
Classically, attempts to control diseases in plants consisted of coating plant surfaces with toxic substances which prevented pathogens, primarily fungi and bacteria, from entering the plant through direct penetration and/or through natural openings such as stomata. Later, systemic chemical fungicides were developed which killed fungi upon infection of the plant.
Many of these chemicals used in plant disease control, however, are enviromentally damaging. Since these chemical control methods require the application of toxic substances to plants, these chemicals find their way into human and animal food and pollute waterways, often causing pathology to fish, birds, and other wildlife. Additionally, many chemicals are toxic for only a limited range of pathogens, requiring the application of multiple chemicals in order to achieve broad protection. Chemicals may also have to be reapplied during a growing season if the chemicals are washed off the plants during a rain.
A second method of controlling pathogens is through the use of disease resistant cultivars of plants. This method has not proven to be totally satisfactory because disease resistant cultivars may not produce the highest yield or highest quality crop compared to non-resistant ones. Also, because many pathogens exist as distinct strains, a cultivar which is resistant to one strain of a pathogen may not be resistant to a different strain.
Recently, researchers have determined that plants have a system for disease resistance whereby systemic acquired resistance can be induced. Induced Systemic Resistance ("ISR") has been induced by prior inoculation with pathogens, nonpathogens and microbial metabolites.
ISR has been induced in a great many plant species, including cereals such as barley, corn, oat, rice, and wheat, cucurbits such as cucumber, muskmelon, and watermelon, legumes such as bean, cowpea, pea, and soybean, solanaceous plants such as pepper, potato, tobacco, and tomato, fruits such as pear, grape, peach, plum, strawberry, and apple, and other plants such as beet, cotton, coffee, radish, carnation and Douglas fir, to protect the plants from a variety of leaf and root pathogens. ISR is reported to be a broad, non-specific form of immunity whereby induction of 5 immunity to one pathogen, for example a bacteria, may result in immunity to a great variety of pathogens, such as other bacteria, viruses, and fungi.
ISR is reported to be mediated by activation of multiple mechanisms for disease resistance. One such mechanism is the accumulation of low-molecular weight antimicrobial substances (phytoalexins) at and immediately around sites of infection. Phytoalexins accumulate rapidly after infection or stress in plants sensitized to respond by prior infection.
Other components of the ISR complex include the accumulation of antimicrobial agents and formation of physical barriers such as, lignification, suberization, formation of callose and papillae, accumulation of agglutinins, enzyme inhibitors and hydroxyproline-rich glycoproteins.
Another mechanism for ISR in plants is believed to be the production of hydrolytic enzymes (such as chitinases and .beta.-1,3-glucanases), other pathogenesis related (PR-) proteins, and anionic isozymes of peroxidases. It has been reported that at least 12 such proteins accumulate in symptomatic tissue extracts in cabbage following challenge with pathogenic Xanthomonas campestris pv. campestris, the causative organism of black rot.
Plant chitinases are reported to be potent inhibitors of fungal growth and in combination with .beta.-1,3-glucanase attack a number of fungi. Chitinases also possess lysozyme activity for hydrolyzing peptidoglycans present in bacterial cell walls. Chitinase and .beta.-1,3-glucanase are coordinately induced in a number of plant tissues by pathogen attack and elicitors.
Peroxidases, which generate H.sub.2 O.sub.2 and oxidize phenols, are important in lignin biosynthesis. Enhanced peroxidase activity has been found in immunized cucumber, muskmelon, tobacco and watermelon plants.
ISR can be transmitted by asexual means of propagation such as tissue culture and grafts. ISR is systemic, whereas with non-systemic chemicals applied to plants, areas of the plant not covered with the chemical may not be resistant to infection.
Unfortunately, because many pathogenic organisms require a wound in order to guarantee inoculation of the plant, current methods of inducing ISR in plants have proven unsatisfactory. Inoculation today is achieved by the labor intensive method of individually injecting or wounding a plant so that non-pathogenic organisms can enter to induce ISR. This method has proven to be impractical and not always as effective as required for modern large scale agriculture.
A list of publications relating to the Background of the Invention or of interest is provided herein below under "references".
It is evident from this background review that inducing immunity in plants presents a serious problem and that, though numerous methods have been proposed, no satisfactory solution has yet been found. This invention contributes to the solution of this world-wide problem by inducing natural defense mechanisms with a novel, safe, reliable and convenient formulation which is applied to the target plants.